The big plus of tack is that it is quick and easy. You roll a little ball of the stuff, poke it onto the bottom of your box, press the specimen onto it, and voila! -- you have a micromount.
There are several negatives. The worst is something I have seen a fair amount in older material. The tack contains oil of some kind and it will migrate into and onto the specimen. Once this happens, I know of no way to remove it -- the only answer is to shed a tear and toss the horrible mess away. The other negative is that tack can dry out and let go of the specimen.
Of course all tack is not created equal. The old blue stuff seems to be particularly nasty as far as releasing oil as well as getting hard as a rock. The newer stuff (as sold currently by Shannon and Sons) is grey and certainly seems much better, but only time will tell.
I sometimes use tack to "park" specimens on that have been trimmed and that I am unable to mount immediately. I view these as "time bombs" -- it is only a matter of time before something undesirable happens. I am taking a certain risk. I also use tack for material I am giving to other collectors. I assume they are wise enough to remount the material permanently within a short time frame.
Another problem with tack is that you can break really delicate material pressing it onto tack. When I have a really delicate piece, it will refrain from "parking" it on tack and make an immediate permanent mount. No force is required to set a delicate piece onto a peg with a drop of glue onit.
I am suspicious that all glues and all plastics will break down given enough time. However I am also convinced that mounts made using glue and pegs will long outlast mounts made with tack. Mounts with labels attached using glue (as described elsewhere) will definitely outlast mounts using stick-on labels.
So are people using tack simply lazy? Well, in some cases they probably are.
In the final thinking about all of this, it is important to remember that this is just a hobby. We are doing this for fun. There are no rules and no enforcement committees. It is your collection and you can do whatever you want. It is important to understand the pros and cons of whatever methods you decide to use. I will only say this -- you will never regret aiming for quality in whatever you do.
Tom's Mineralogy Info / tom@mmto.org